(1) FIELD OF THE INVENTION
This invention relates to internal combustion engines, and more particularly it is concerned with a four-cycle internal combustion engine of the type having suction valves and exhaust valves which is equipped with an automatic decompression device for reducing torque and power required for starting the engine by reducing the compression pressure in the cylinder at engine startup.
(2) DESCRIPTION OF THE PRIOR ART
A proposal has been made to use an automatic decompression device which is intended for use with engines of all the types, as discloses in Japanese Utility Model Publication No. 20486/81. This device is capable of improving starting characteristics by automatically opening a valve (an exhaust valve, for example) in a compression stroke at engine startup to thereby decompress the combustion chamber.
In the aforesaid device, a camshaft is journalled at one end portion thereof by a bearing mounted at the crankcase, and a cylindrical holder equipped with a centrifugal cam portion for decompression is fitted over the camshaft and secured in place between the bearings and cam. In this arrangement, two bearings by which the camshaft is journalled are spaced apart from each other a large distance, making it impossible to obtain a compact overall size in an engine. Moreover, in this device, a bending moment caused in the camshaft increases and makes it necessary to increase the strength of the camshaft by increasing its thickness. This inevitably increases the weight of the device.
In view of the aforesaid problems, no attempts have hitherto been made to equip a motorcycle engine with a centrifugal type automatic decompression device. Many motorcycle engines have had a decompression mechanism of low efficiency which is either manually actuated or linked to a kickoff starting device. Thus, there has been a demand for an automatic decompression device which is light in weight, compact in size and reliable in performance.